The invention relates to a sensor for detecting the instantaneous concentrations of a plurality of gas constituents in a gas, in particular for exhaust-gas measurement, and combustion exhaust gases from incineration plants for fossil or biological fuels or waste or from combustion engines. For example, automobile engines operating on the diesel principle contain harmful substances which represent environmental pollution. In particular, nitrogen oxides, as toxic and environment-endangering substances, are at the center of public interest and should be removed as completely as possible from the combustion exhaust gases in order to avoid environmental pollution. Examples of such nitrogen oxides are NO, NO2, NO3, N2O3, N2O4 and N2O5. These are known from the textbook literature, but NO and NO2 are of particular importance.
A diesel exhaust gas can, according to the reference Kraftfahrtechnisches Taschenbuch, Robert Bosch GmbH, 1991, p. 513, have the following compositions:
HC denotes hydrocarbons, H2O is in the form of steam, and the concentrations are based on volumes. The temperature is 100-750xc2x0 C., and the pressure, which is not quoted, is assumed to be from 1 to 1.05 bar. SO2 may additionally be present.
The nitrogen oxides NOx are frequently reduced using ammonia or ammonia-releasing substances, such as urea, ammonium carbonate, ammonium hydrogen-carbonate, ammonium cyanate and others. Urea can be fed in, for example, in a 30% strength aqueous solution, and NH3 in the form of a gas. The reduction proceeds in accordance with the following reaction equations:
4 NO+4 NH3+O2xe2x86x924 N2+6 H2O
NO+NO2+2NH3xe2x86x922 N2+3 H2O
2 NO2+4 NH3+O2xe2x86x923 N2+6 H2O
In order to convert the nitrogen oxides as completely as possible, an equimolar or higher proportion of ammonia is advantageously added. The molecular weights are 17 for NH3, 46 for NO2, 62 for NO3 and 54 for (N2O5)/2. For a stoichiometric reaction, about three times the proportion by weight or a single proportion by volume of ammonia, based on NOx, is needed. If a 3% excess of ammonia is assumed with the abovementioned composition with on average 2000 ppm of NOx, 60 ppm of ammonia remain in the exhaust gas. If this is not to be exceeded, the measurement must still be able to detect 60 ppm with a reliability of 10 ppm. The measurement must still function reliably at exhaust gas temperatures of from 100 to 750xc2x0 C. Furthermore, soot or ammonium salt dusts must not interfere with the measurements, and the sensor must not be affected by corrosion caused, for example, by sulfur oxides.
Incorrect measurements entail the risk of a relatively large excess of one of the environmental pollutants ammonia or nitrogen oxide being present in the exhaust gases. Both are very undesired in the environment. Attempts are being made to reduce the concentration of nitrogen oxides to below the legally permissible level and at the same time not to add any excess ammonia.
In DE-A-3 721 572, the nitrogen oxide (NOx) concentration in the exhaust gases of an engine are measured for selective catalytic reduction of NOx and, depending on the measured NOx concentration, fed to a catalyst NH3 for conversion. At least 75% of the regulation of the amount of NH3 is effected by the engine load data, while the remainder is regulated depending on the NOx concentration measured in the exhaust gases. It is hoped that this will improve the sluggish regulation of the NH3 supply known hitherto by measuring the NOx concentration after the catalyst in non-steady-state operation, where the power and speed of the engine and thus the NOx concentration in the raw emission from the engine change rapidly. This reaction time is given as about 1 minute, while the addition of ammonia remains constant for a different amount of exhaust gas and a different exhaust-gas composition, since the sluggish sensor has not yet detected the change.
EP-B-0 447 537 furthermore discloses that, for operation of an oxidation catalyst, the NH3 portion must be as low as possible in order to achieve the catalytic action for destruction of dioxins.
The journal xe2x80x9cSensors and Actuatorsxe2x80x9d, B4, 1991, page 530, gives a response time of 30 s for the commercial SOLIDOX-NH3 system. Here too, the measurement time is too long for adequate regulation of a non-steady-state operating mode.
U.S. Pat. No. 2,310,472 discloses analyzing automobile exhaust gases by burning them catalytically and measuring the temperature increase as an increase in the resistance of the combustion catalyst. This is carried out by means of a Wheatstone bridge. The catalyst used is a cerium oxide-coated platinum wire or a filament of catalytic material, such as platinum. The addition of ammonia for the catalytic NOx reduction of the automobile exhaust gas is not mentioned.
U.S. Pat. No. 2,583,930 discloses analyzing combustible gases or vapors by burning them catalytically. The catalyst proposed is a platinum wire or a platinum/rhodium wire whose change in resistance is measured, again using a Wheatstone bridge. The particular advantage here is the avoidance of drift.
EP-A-0 591 240 discloses a sensor for measuring ammonia. The sensor is sensitive to one gas constituent through a thin layer of platinum or palladium applied to an oxide surface. A combustion reaction, for example, is catalyzed, and the primary signal is the electrical resistance of a semiconductor. At least two sets of electrodes attached to the semiconductor at different distances enable an impairment in the function to be recognized.
U.S. Pat. No. 3,586,486 discloses an analysis of an automobile exhaust gas in which the catalyst employed for the combustion is platinum in the form of a platinum black film in a thickness of 0.0508 mm (0.002 inch). The resistance of the catalytic resistance element depends on the temperature, and the measurement is carried out using a Wheatstone bridge.
The fact that the above-described sensor does not work satisfactorily is noted by U.S. Pat. No. 4,197,089, which proposes a WO3 film as sensor. This is sensitized for NH3 by a small amount of a platinum catalyst. The platinum catalyst is in the form of a thin layer under the WO3 film. The change in resistance is measured by means of a Wheatstone bridge, with the resistance of the WO3 film dropping owing to the reducing agent H2S or NH3.
DE-A-4 117 143 discloses analyzing the proportion of NH3 in automobile exhaust gases from diesel engines. NH3 is oxidized catalytically, with the increase in the temperature of the gas caused by the evolution of heat being taken as a measure of the NH3 concentration. The catalyst is located within the honeycomb channel, or alternatively a gas sub-stream can be taken. Furthermore, stoichiometric feed of NH3 at constant full-load operation is disclosed, but otherwise cycled, super-stoichiometric addition is the subject matter of this publication.
DE-C-3 543 818 describes how an electrochemical ZrO2 cell functions on this basis for measuring the oxygen concentration in gases. Another, likewise known process for measuring the oxygen concentration is disclosed in an information sheet from Dittrich Elektronik, Bahnhofstrasse 67, 76532 Baden-Baden, which is commercially available together with the oxygen measurement cell.
The slowness of the sensors is a recurring problem since too few measurement data or an excessively long measurement time make regulation of the non-steady-state operating mode more difficult.
Significantly shorter reaction times are obtained with a sensor with which the instantaneous concentrations of oxygen, ammonia and nitrogen oxide in combustion exhaust gases are determined simultaneously. In the sensor, three different zones are provided, in one of which the oxygen concentration is measured and in the other two the concentrations of NH3 and NO are measured, preferably on the basis of the oxygen consumption in the partially selective reaction of ammonia or nitrogen oxides and ammonia with oxygen. An increase in temperature caused by exothermic ammonia or nitrogen oxide combustion can also give a measure which is suitable for measurement of the gas concentration. The measurement of small ammonia concentrations is of particular importance here. For example, the ammonia concentration of about 60 ppm present in combustion exhaust gases should be measured continuously with an accuracy of 10 ppm in order that, on the basis of the measurement results, the metering of the ammonia takes place in such a way that significant environmental damage is not caused.
In addition to measurement of O2, NH3 and NO, it may be advantageous to measure the CO concentration in a fourth zone. A suitable catalyst here is, for example, gold.
A particularly suitable sensor is one whose second zone contains a reaction catalyst at least for partially selective reaction of NH3 with O2, whose third zone contains a reaction catalyst at least for partially selective reaction of NO and NH3 with O2 and whose first zone is designed to measure the partial pressure of O2.
The temperature increase can also be set in relation to the ammonia concentration by measuring the heat of combustion on a reaction catalyst, which results, for example, in an increase in the electrical resistance. It can be assumed that the suitable reaction catalysts are electrical conductors whose electrical resistance increases in a readily measurable way with increasing temperature.
The arrangement of the zones on a common substrate has the advantage that the same conditions exist for all three zones. This is particularly important if the substrate is designed to be oxygen-conducting and, together with the zones, delimits gas-filled chambers in which a reference pressure of oxygen has been established. The use of a common substrate establishes in this case uniformity of the oxygen concentrations. This is because the conditions can change along the gas stream. Thus, the content of O2, NH3 and NO is relatively high at the beginning of the catalyst where little reaction has as yet taken place, while the contents toward the end of the catalyst drop considerably, as intended. In accordance with the invention, the contents can be measured closely adjacent to one another at the same point.
In order to prevent sooting of the sensor and to favor dissociation of the oxygen molecule, a heating device may be provided. The heating device is advantageously of such dimensions that a temperature of 500-900xc2x0 C., preferably 700xc2x0 C., is achieved in the first to third zones.
A particularly suitable sensor is a construction having a substrate of a material in which O2 can be transported in the form of ions. The substrate is bonded on the one side to a reference layer of a reference material and likewise has on the other side a layer of the reference material, in which chambers are arranged, where the chambers are covered and overall sealed in a gastight manner by sheets of a material in which O2 can be transported in the form of ions, reaction catalysts being arranged on the sheets. Under the action of current, the reference material of the reference layer causes the transport of oxygen in the form of ions through the substrate into the chambers and thus represents a reference quantity for the partial-pressure gradient of the oxygen in the chamber compared with that of the gas stream to be measured. An O2 partial pressure is measured in all three zones.
The sensor is advantageously provided with means for recording the voltage between the reaction catalysts and the reference layer.
The sheets are advantageously fixed to the substrate by means of a glass adhesive. The glass adhesive is not oxygen-conducting, even at the high operating temperatures, and seals the chambers.
The present invention furthermore relates to the use of the sensor according to the invention in a catalyst system in which NH3 is fed in for reduction of nitrogen oxides in combustion gases, where the sensor is arranged at least at the catalyst outlet and/or in the catalyst after the NH3 feedpoint. In addition, a sensor may advantageously also be installed immediately before the catalyst and/or before an NH3 feedpoint. In contrast to the catalyst systems known hitherto, pointwise, reaction-quick measurement of the nitrogen oxides in the combustion gases enables finely metered NH3 feed, so that an excess of NH3 does not exit from the catalyst system.
Thanks to the sensor according to the invention, a further catalyst system can be achieved which, after a DeNOx catalyst, additionally contains a dioxin-converting catalyst part. The prerequisite for conversion of dioxin in the catalyst is that the NH3 concentration remains below a certain limit.
The NH3 feed is advantageously regulated on the basis of the measurement results from the sensor according to the invention.